Dry lubricant composition and a



March 31, 1964 c. s. OLIVER ETAL 3,127,346

DRY LUBRICANT COMPOSITION AND A METHOD FOR ITS APPLICATION Filed March 25, 1960 Wear Rate vs. Composition for M052 SW52. Mixtures %by Weight I I I I l I I l l I I l l l I I l I I l l I I I I wear Rate Mg, Hour Inventors: Curtis 3 Oliver Arthur J. Ha/tner,

The/r Attorney.

United States Patent 3,127,346 DRY LUBRICANT COMPOSITION AND A METHOD FOR ITS APPLICATION Curtis 5. Oliver and Arthur J. Haltner, Schenectady, N.Y., assignors to General Electric Company, a corporation of New York Filed Mar. 23, 1960, Ser. No. 19,978 2 Claims. (Cl. 25225) This invention relates to solid lubricants otherwise referred to as dry lubricants and more particularly to the combination of lamellar metal compound lubricants together with other sulfides as an improved solid lubricant. This application is a continuation-in-part of our copending application Serial No. 830,482, filed July 30, 1959, now abandoned, and assigned to the same assignee as the present application.

With bearing requirements progressing towards operating limits of increased temperature and load carrying ability, the field of lubrication has necessarily been extended to include these operating parameters. A type of lubricant to which this invention is more particularly directed is a solid or dry lubricant as differentiated from liquid and/or grease type lubricants. A good example of a solid lubricant and bearing material is nylon or Teflon which are employed as bearings in relatively light applications. Another example of a well known solid lubricant is graphite which, in flake or powder form, is maintained between relatively moving surfaces as a lubricant material. Certain inorganic compounds, i.e., the sulfides, disulfides, selenides and tellurides of such metals as molybdenum, tungsten, titanium, zirconium, uranium, etc., are characterized by a laminated or plate-like crystal structure in which the metallic atoms are arranged in a single common plane while the non-metallic atoms are attached to the metallic atoms to form layers on both sides of this plane. The non-metallic atoms in each of the crystals in the sandwich structure have very little attraction for each other so that these crystals will slip readily with respect to each other under the action of low shearing forces. Moreover, the non-metallic atoms have an affinity for adjacent metal surfaces so that the crystals attached to such surfaces will exhibit very strong resistance to the action of forces normal to the direction of shear. Consequenty, these compounds have excellent lubricating properties and also have excellent anti-seizing properties. These lubricants will be hereafter referred to as lamellar metal compound lubricants. Most promising of these lubricants are molybdenum disulfide (M05 and tungsten disulfide (W8 It has been found, however, that M08 and W5 more particularly, have more pronounced disadvantages as dry lubricants including, first, they have a high wear rate and thus wear quite rapidly with continual replenishment being necessary; second, as the lubricant wears, large amounts of the material are accumulated between moving surfaces and may in some instances cause jamming or other undesirable effects. Furthermore, these lubricants have poor load carrying ability and their use is thereby restricted to light applications.

Accordingly, it is an object of this invention to provide an improved dry lubricant of the lamellar metal compound type in which the metallic atoms are arranged in a single common plane while the non-metallic atoms are attached to the metallic atoms to form layers on each side.

It is another object of this invention to provide a combining material with lamellar metal compound lubricants to improve the characteristics thereof as dry lubricants.

It is a further object of this invention to improve the wear characteristics of M05 and W8 particularly as dry lubricants.

It is still another object of this invention to increase the lubricant characteristics of M05 and W5 particularly by extending the load carrying capabilities and temperature range thereof.

Briefly described this invention provides additives for the lamellar metal compound lubricants as dry lubricants which increase the load carrying ability or decrease the wear rate while not adversely alfecting the lubrication characteristics.

These and other advantages, features and objects of this invention will be better understood when taken in connection with the following description and the drawing in which:

FIG. 1 is a curve representing SnS mixtures.

At present, a most commonly described concept concerning the choice of a solid lubricant is that it have a low shear strength. Such a concept assumes a mechanism of lubrication whereby the particles of solid lubri cant crystals are firmly held between moving surfaces and that these crystars are sheared as their surfaces move. This lubricating mechanism apparently requires a continual formation of fresh surfaces of the crystal lubricant. With respect to the fact that a solid lubricant has a frictional behavior as that of a solid sliding over itself, for this to happen, the material from the lubricant surface must attach to the surface being lubricated. Then, the resulting coetficient of friction on all surfaces that are successfully lubricated should be the same. For shearing and sliding to occur easily in the lubricated film, the crystals of solid lubricant should be orientated with the planes of easy cleavage along the sliding direction. Such preferred orientation occurs in the case of films of the lamellar metal compound lubricants as described.

A typical presentation of the lubricating properties of, for example, a M08 film alone is given in Table I. M08 is one of the more widely employed and satisfactory dry lubricants of the lamellar type.

TABLE I Frictional Properties of Mo-S Films the wear rate of M05 Max. Load Carried, Friction, g kg.

Coefficient Disc Metal or The values in Table I and the following tables were obtained by forming a solid lubricant film on various surfaces by rubbing. After a continuous film was formed on the metal surface, the M05 which provided the film was substituted by a further steel surface in the form of a A; inch diameter hemispherical rider, and the coefiicient of friction was measured between the rider riding and the film formed on the other metal surface. Tests were made in air, at room temperature and at a relative humidity of 30-35%. Speed was about 2 feet per second. It is noticeable that on chromium, and cast iron very high loadings were possible.

It has been discovered that the addition of various sulfides results in marked changes in the lubricating characteristics of the lamellar metal compound lubricants. A typical example of an additive sulfide is SnS The description thus proceeds relative to MoS -SnS mixtures as illustrative of other lamellar metal compound lubricants and other sulfides. While SnS is a good additive,

it is not the optimum additive as will be hereafter noted in Table VI, but is chosen for purposes of description.

The addition of a small amount of stannic sulfide, SnS to the lubricant film changes the situation drastically. For example, a molybdenum disu-lfide film on mild steel can carry a load of only a few hundred grams, but when as little as by weight of stannic sulfide is present in a film to comprise by weight 90% M08 and 10% SnS the load carrying ability may be increased many times. In the case of chromium, the breaking strength of the film was not exceeded. Soft surfaces, such as copper and tin, show a relatively low breaking strength even with the stannic sulfide present.

The crystal structure of stannic sulfide is that of an inorganic layer lattice. This is a structure which is common to a number of inorganic solid lubricants, however, an investigation of frictional behavior of stannic sulfide indicates that this material has a high coefficient of friction on a large number of metallic surfaces, and thus is a poorer solid lubricant than M08 Typical behavior of a solid form of SnS sliding on various surfaces is shown in Table II as follows:

TABLE II Coefiicient of Friction Data for SnS Pellets Surface Load, g. a

1. Copper 711 .55 2 Brass 1, 237 .30;|;.05 3. Stainless SteeL- 417 .9 4. Nickel 605 62 5. Mild Steel. 1, 047 55:l:.02 6. 1040 Steel 625 59:1;02

SnS; is basically a poorer lubricant than M08 A sliding pellet of mixtures of SnS and M08 indicates a much lower coefiicient of (friction than that of an SnS pellet alone as indicated in the following table:

While Tables II and III indicate the coefiicient of friction of pellets rubbing on a surface, they also are illustrative of the feature that adding SnS to M05 where SnS is a poorer lubricant than M08 does not greatly affect the coefiicient of friction of M08 alone. In addition, it was observed that the wear rate of the pellet was markedly low compared to M08 alone.

It was found that the above advantages of MoS -SnS mixtures were true when applied to films. Good results were obtained using stannic sulfide (mosaic gold), about 97 or 98% SnS Large crystals were ball milled to give fine powder for pressing pellets. Qualitative spectrographic analysis indicated the following: Fepresent, Sipresent, P\bpresent, Al-present, Mg-trace, Cu-- trace Ag-trace, Ni-trace, Mn-trace, Mo-present. The X-ray powder pattern contained lines characteristic of hexagonal SnS together with a very weak pattern for tetragonal SnO Typical frictional values for various compositions of MoS /SnS are given in Table IV as follows. Compositions are 100% by weight with various percentages (as given) of M08 and SnS by weight providing the 100% mixture.

TABLE IV Frictional PIOPEIIIES of Mos /SnS F zlms Coetficient Max. Load Composition, Mos lsnsz Disc Metal of Carried,

Friction, [1 kg.

Mild Steel .058 1. 46 d0 .047 2 81 1090 Steel .086 .48 Stainless Steel.-. 17 .39 017 6.06 040 3. 10 011 24 16 .38 12 25 084 2 15 4 The nature of the wear process for M03 is favorably altered by the addition of SnS A substantial portion of the wear of M08 pellets appeared to be or to occur by the formation of a thick M05 track on the surface to which the film was attached. This film built up rapidly and was accompanied by shearing of the film to produce large flat platelike particles in the wear area. With SnS present, a much thinner track or film was built up on the copper surface, about the thickness of M05 alone, and the wear particles besides being smaller in bulk are more comparable in size to powders in the starting ma terial. This prevention of a thick buildup is a very useful property, since in some applications the buildup of M03 is suificiently great to fill the clearance between moving parts and produce jamming. It also seems clear in the findings, that M08 is the actual material providing the solid lubricant and that the role played by SnS is to alter the surface to such a state that M08 may perform its lubricating function more readily. Therefore, it may be desirable in some instances to prepare a surface for the addition of M08 by applying a film of SNS; or other sulfide previous to applying M08 For example, lubrication of 1090 steel, Table IV, was increased over an application of M08 SNS film by providing a film of SnS and then superimposing a film of M08 In all instances, however, the SnS should have a minimum of stannic oxide present or, alternatively, the oxide present, in any ratio of M08 and SnS should be of a small amount so as not to affect lubrication.

While this specification thus far has been directed to SnS as an additive, the description thereof is common to other materials which may be employed as additives. It was discovered that not only may various additives be used in addition to SnS but that also a common feature was present among these additives. Generally speaking, it is believed that a sulfide is an effective additive to a lamellar metal compound lubricant provided that it is less thermodynamically stable than the sulfide of the surface material to which it is attached. For example, if a sulfide additive is to be employed with M08 to lubricate a steel shaft, then the additive must be thermodynamically less stable than FeS. An alternate Way of stating this is that those sulfides which appear to be effective additives have a lower negative free energy of formation than FeS, and those which are ineffective as an additive have a higher negative free energy of formation than FeS. This action may be explained as follows. When a lubricant film, for example M08 on a steel shaft, alloy or otherwise, breaks down from various reasons, metal to metal contact results with a high local temperature of approximately 300400 C. These conditions favor a reaction between the additive and the metal surface more readily than between the additive and M08 thus resulting in FeS being produced on the shaft surface which effectively repairs the film. The lubricating mechanism may be that of sliding upon the FeS reaction film with the shear in this film contributing to the frictional force. The contribution to the total frictional force may be small providing the area of such reaction film is also small. The important property of the reaction film then is to prevent rapid propagation of film failure once a weak spot occurs. It is also believed that there is good adherence between the sulfide reaction film and the M lubricant. Therefore, the M05 may spread out more readily to cover the break. This reasoning is supported in that M08 showed excellent and rapid film forming on chemically sulfided surfaces.

There are various lamellar metal compounds employed as dry lubricants, in addition to M08 for example, W8 ZrS and TiS which are generally equivalent in thermodynamic stability to M08 Mixtures of these lubricants, therefore, do not provide the optimum benefits of the mixtures of this invention wherein the additive sulfide is substantially less thermodynamically stable, and which are additives in nature. However, the obvious chemical similarity of these lamellar metal compounds indicates that they also become better dry lubricants when combined with a sulfide additive. This is illustrated in following Table V.

TABLE V Frictional Properties of W8 Films on Mild Steel Load Additive p Carried,

None 0.12 0.7 None 0. 11 1.1 Pbs 0.035 5.3 10% AgS 0. 020 8.1

The following Table VI sets forth numerous sulfides employed as additives to M08 as a dry lubricant. The particular technique for studying these films was the same as those employed previously. Mild steel was employed as the substrate in all instances. Tests were made in air, at room temperature, and at a relative humidity of about 30-35%. The additive concentration was standardized at 10% by weight of the pellet, and the rider in each instance was a inch diameter steel hemisphere. Compacting pellet pressure was about 26 tons per square inch for pellets in this as well as other tables.

The values of Table VI are indicative, when compared to that of MoS on mild steel, that the load carrying ability of an MoS additive film is much greater than M05 without an additive.

It also should be understood that other and fluid compounds may be employed as indirect additives where the compounds may decompose or otherwise react to produce an additive compound, in accordance with the teachings of this invention, to be less thermodynamically stable than the parent lubricant and the sulfide of the surface material upon which it is to be employed. For example, BaS may react with water or moisture to produce H 8 which is an additive albeit a fluid.

The crystal structure of the additive is also a factor influencing the usefulness thereof. For example, in Table VI, it is noted that the black modification of I-lgS 6 is a less effective additive for M03 whereas the red modification is an outstanding additive.

The following Table VII indicates M05 with selected sulfide additives which decrease lubricant wear rate.

1 Percent by weight.

Copper was chosen as exemplary for its good film forming properties. Additives lower the wear rate of the film considerably. Several operations on other metal surfaces, for example steel, indicate that wear rate is proportionately reduced.

The amount by weight of additive may vary for particular circumstances. For example, considering Wear rate, only a small percentage is necessary since a progressively higher wear rate is encountered when the amount of additive increases over that amount provid ing a minimum wear rate. Generally, a positive amount from about 0 to 50% provides a reduced wear rate from a minimum wear rate at about 2% and increasing at 50%. The wear rate at 50% additive, however, is still lower than M08 alone. The wear rate of various mixtures of, for example, M05 and SnS is given in FIG. 1. Referring now to FIG. 1, it is understood that, as before mentioned, only a small amount of SnS or other additive is needed to markedly decrease the Wear rate, and additional amounts may be added to provide a wear rate which is still below that of M08 alone. The wear rate factor is important because it may be decreased substantially without substantially affecting the lubrication characteristics of M08 alone. Curves for other mentioned additives are similar.

The dry lubricant of this invention may be applied to a given surface by means well known to those skilled in the art. One preferred method is to employ the lubricant as a solid stick type of lubricant because in this form the stick may be made up without a binder which ordinarily limits the lubrication characteristics of stick lubricants. Other means of application are numerous including, for example, providing a film of lubricant from a liquid mixture by spraying, dipping or otherwise coating a surface to be lubricated.

It has been discovered that a lamellar metal compound lubricant film can be applied directly to a given surface Without the use of the binder by an application process including burnishing.

For example, in the practice of this invention, to apply a film to a shaft, the shaft may be mounted in a lathe or other similar rotary apparatus and a small diameter pellet of the lubricant material, suitably attached to a holder is forced against the shaft and moves along the shaft to provide a film of desired width. The force maintained between the lamellar metal compound lubricant and the surface to which the film is to be attached should be greater than the force usually contemplated in the term rubbing and in most instances is great enough to be just under the crumbling or breaking strength of the lamellar metal compound lubricant. Only a few passes 7 along the shaft are necessary, since a maximum buildup of the lubricant film is soon reached and no additional benefit is gained by further application.

Burnishing of the surface after application of the dry lubricant provides a marked improvement including that the burnishing action provides a much stronger bond between the lubricant and the surface, and that the film evidences long life with actual improvement in lubricating characteristics. The term burnishing as employed is similar to the term as employed in the metal working art. Specifically, and for example, a dry lubricant film was applied to a shaft as described. Thereafter, a /8 inch diameter steel ball was pressed into the shaft just sufliciently to metal work the surface and one pass made over the dry lubricant area.

The shaft of a very small fan motor, such as employed to maintain air circulation in freezers, having an applied burnished film of M08 has been in substantially continuous operation over a period of a year with no additional lubrication provided or necessary.

A preferred substrate for this burnishing method has been discovered to be chromium which is generally impervious, crack free, and maintained in a chemically clean state until just prior to the application of the lubricant film.

EXAMPLE I An M03 film was applied to a chromium plate as described above from pure pellets, a molykote microsize M08 A inch diameter pellet with an applied load of 2 kg. was rubbed on the plate at a speed of 100 feet per minute. Compacting pressure of the pellet was about -15 tons per square inch. A thin haze of MoS formed almost instantly followed by a slow increase as rubbing continued.

The film as obtained in the above example was burnished and tested by having a /8 inch diameter steel hemisphere loaded gradually from zero with 5 0100 gram increments while running on the film. This rubbing geometry imposed a severe test on the M08 film. Apparent areas of contact are quite small and in many instances, apparent pressures on the order of 50,000 p.s.i. are observed. This indicates a case of extreme pressure lubrication.

EXAMPLE II A film acquired in accordance with the teachings of Example I was utilized under tests as above described for a total time of 30 hours. During this run, exposure was made to water vapor in the range of partial pressures from 0 to 25 millimeter of mercury. At the termination of this test, the film was still intact with a coetficient of friction in dry nitrogen of .03.

EXAMPLE III Similar tests made on M08 containing an additive of about 10% by weight of SnS carried a load of 6.1 kg. with the film remaining intact. The ultimate load carrying ability of this film could not be determined. The coefficient of friction was .017.

It is understood that the object of this invention is attained by the combination of lamellar metal compound lubricants with sulfide additives single or plural. In all instances, as opposed to numerous well known additives,

the sulfide additives of this invention provide improved lubrication by way of increased load carrying ability, decreased wear rate and corresponding reduced film thickness without substantially affecting the coefiicient of friction. Therefore, the prime disadvantage of film thic. ness and high wear rate is minimized.

In general, for the examples as given and for FIG. 1, the pellets have a contact surface of about inch and were employed with 1.8 kilogram loading, unless otherwise indicated. Sliding speed was about 2 ft./sec. Copper plates were from commercial spinning grade copper sheet.

While other modifications or variations of this invention that may be employed within the scope thereof have not been described, the invention is intended to include all such as may be embraced within the following claims. In the claims, the term lamellar metal compound lubri cants refers to the general description of such lubricants as given in column 1 of this specification.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. A method for applying a dry lubricant to a metal surface capable of forming a sulfide comprising the steps of:

(a) coating the portion of the metal surface to be lubricated with a film composed of at least one material chosen from the group consisting of Sb S PtS, red HgS, Ag S, PbS, FeS, Ti S Cu S, CuS, Au S, Bi S S, SnS black HgS, T1 8 Cr S CaS, BaS and CdS, and (b) overlaying said film with a coating of at least one material chosen from the group consisting of M05 W52, ZI'SZ, and Tlsz,

the material chosen for said film being less thermodynamically stable than the material chosen for said coating and also less thermodynamically stable than the sulfide of said metal surface. 2. A dry lubricant for application between juxtaposed surfaces at least one of which is a metal capable of forming a sulfide, said lubricant comprising a mixture of at least one lamellar metal compound lubricant chosen from the group consisting of M08 W5 ZrS and TiS with at least one additive chosen from the group consisting of Sb S PtS, red HgS, Ag S, PbS, FeS, T i 8 Cu S, CuS, Au S, Bi S S, SnS black HgS, T1 8 Cr S CaS, BaS and CdS, provided that any additive employed must be less thermodynamically stable than the sulfide of the metal surface being lubricated and less thermodynamically stable than the lubricant material chosen.

References Cited in the file of this patent UNITED STATES PATENTS 2,156,803 Cooper et al. May 2, 1939 2,367,946 Kaercher J an. 23, 1945 2,420,886 Laifoon May 20, 1947 2,421,543 Cook June 3, 1947 2,609,342 White et al Sept. 2, 1952 2,622,993 McCullough et al. Dec. 23, 1952 FOREIGN PATENTS 794,982 Great Britain May 14, 1958 

1. A METHOD FOR APPLYING A DRY LUBRICANT TO A METAL SURFACE CAPABLE OF FORMING A SULFIDE COMPRISNG THE STEPS OF: (A) COATING THE PORTION OF THE METAL SURFACE TO BE LUBRICATED WITH A FILM COMPOSED OF AT LEAST ONE MATERIAL CHOSEN FROM THE GROUP CONSISTING OF SB2S5, PTS, RED HGS, AG2S, PBS, FES, TI2S3, CU2S, CUS, AU2S, BI2S3, S, SNS2, BLACK HGS, TL2S3, CR2S3, CAS, BAS AND CDS, AND (B) OVERLAYING SAID FILM WITH A COATING OF AT LEAST ONE MATERIAL CHOSEN FROM THE GROUP CONSISTING OF MOS2, WS2, ZRS2, AND TIS2, THE MATERIAL CHOSEN FOR SAID FILM BEING LESS THERMODYNAMICALLY STABLE THEN THE MATERIAL CHOSEN FOR SAID COATING AND ALSO LESS THERMODYNAMICALLY STABLE THAN THE SULFIDE OF SAID METAL SURFACE. 